DIETARY-PROTEIN AND OR ENERGY RESTRICTION IN MARES - PLASMA-GLUCOSE, INSULIN, NONESTERIFIED FATTY-ACID, AND UREA NITROGEN RESPONSES TO FEEDING, GLUCOSE, AND EPINEPHRINE

Sixteen light horse mares (8 to 9 yr of age; 457 to 579 kg BW) were fe d Bermudagrass hay and a corn/cottonseed hull-based supplement formula ted to contain either 100% (control) or 50% (restricted) of the protei n and(or) energy requirements for maintenance in a 2 x 2 factorial arr angement of treatments. Daily measurements of intake, BW, and plasma h ormones and metabolites were made for 33 d. Plasma glucose, insulin, N EFA, and urea N were measured in hourly samples drawn on d 27, and par allel with an i.v. glucose tolerance test (IVGTT) and epinephrine chal lenge on d 29. Energy restriction increased daily NEFA concentrations (P < .001) and urea N (P = .013), whereas protein restriction decrease d (P = .002) urea N concentrations. These effects of protein and energ y restriction occurred within 24 h and were consistent (day effect, P > .1) throughout the remaining 24 d. Normal meal consumption elevated plasma glucose, insulin, and urea N concentrations (time effect, P < . 08). Plasma NEFA concentrations did not change after feeding in mares fed control energy, but decreased in mares fed restricted energy (ener gy x time interaction, P = .005). After IVGTT, areas under the curve f or plasma glucose and insulin were smaller in mares fed restricted pro tein (P < .05), whereas glucose area was larger in mares fed restricte d energy (P = .009). After epinephrine injection, energy restriction i ncreased the initial magnitude of the NEFA response, but after 50 min, reduced plasma NEFA below pre-injection concentrations (energy x time interaction, P = .06). We conclude that metabolic responses occur wit hin 24 h of dietary changes and that plasma constituents are altered b y protein and(or) energy restriction during feeding, glucose, and epin ephrine challenges.